With the growing realization that the world supply of fossil fuels is gradually being diminished, increased attention is being given to the utilization of renewable sources of energy. The search is now underway for new and better ways to harness the winds, the tides and the radiation from the sun.
In the growing utilization of these and other long-neglected energy sources, new and different energy-conversion means will become practical or more nearly optimum. Thus, for example, while large steam turbines have performed admirably in the conversion of energy from coal or gas, the relatively lower temperatures realized in solar collectors seriously limit the efficiency of this conversion means in a solar energy system.
Other factors which argue against the use of steam turbines in solar systems are the typically smaller scale and the variability in the power levels at which they are ordinarily operated. Vapor turbines tend toward low efficiencies in the smaller sizes due in the main to extremely small inlet volume of the fluid and the narrow speed band over which their efficiencies peak.
Another class of converters, the positive displacement or quasi-positive displacement expanders, might prove better suited to such applications. This class includes the conventional steam engine and various types of lobed or vaned rotary expanders. Most expanders, however, are beset with the problem of irreversible heat transfer which significantly reduces the available energy. In addition, they are limited by the relatively low volume expansion ratios achievable in a practical engine design.
Means have been found for the relief of some of these deficiencies. Superheating the working fluid increases the ratio of specific heats so that for a fixed volume expansion the temperature drop is maximized. In addition, the irreversibilities associated with moisture in the vapor are largely eliminated thereby.
The most efficient types of steam engines in the condensing regime are called uni-flow engines. In these types of engines the hot steam is caused to flow over hot surfaces and the cold wet steam is "blown down" over relatively cool surfaces. This technique minimizes the availability loss otherwise caused by mixing together elements of different temperatures. However, one remaining weakness is that during the recompression of the residual exhaust steam left in the cylinder, heat is irreversibly transferred as the entrained drops are re-evaporated.
Additional improvements in operating efficiencies are essential to the practical application of these engines in the new environment of an energy-conscious society. A promising approach for achieving such an improvement comprises a means for totally expelling the residual exhaust products at the completion of the "blowdown" phase.